1. Field of the Invention
The present invention relates to a gallium ion source, which is used in a focused ion beam system, such as a section specimen preparation apparatus for mask repair, a scanning ion microscope and a transmission electron microscope, and an ion beam etching system for section formation used in a semiconductor inspection apparatus.
2. Discussion of Background
Gallium ion sources that have been industrially used at present comprise a needle electrode having a tip provided with a sharp edge, a reservoir for storing gallium, a heater for heating the needle electrode and the reservoir by Joule heating, and supporting terminals for attaching the needle electrode, the reservoir and heater to an insulating ceramic base. In the known gallium ion sources, the needle electrode and the heater are made of tungsten wires, and the needle electrode and the heater are jointed by spot-welding.
With regard to the operational conditions of the gallium ion sources, it is reportedly preferable that the gallium ion sources are operated at a low emission current and at a low temperature to minimize the energy distribution spread of emitted ions so as to make the beam diameter smaller, which is stated in J. Appl. Phys., 51, 3453-3455 (1980).
Gallium can easily keep a liquid state at a temperature near room temperature since the melting point of gallium is 29.8xc2x0 C. and since gallium can easily keep a supercooling state. From this viewpoint, the operation at room temperature has been dominant in the gallium ion sources. However, when the operation is continued at room temperature, it becomes impossible to carry out stable ion emission since the surface of gallium is contaminated by residual gases in a vacuum and sputtered metals from electrodes, as a result of disturbing the feed of gallium from the reservoir to the tip of needle electrode through side surfaces of the needle electrode.
In order to recover the stable ion emission from an unstable gallium feed, an operation, so called flashing, is carried out to temporarily raise the temperatures of the reservoir, the needle electrode and the gallium to about 600xc2x0 C. or higher so as to evaporate contamination for cleaning. The flashing operation is normally carried out about once in 50 hr when the gallium ion sources are operated under an operational vacuum circumstance of about 1xc3x9710xe2x88x927 Torr, which is a typical operational circumstance. The life of the gallium ion sources, which have been industrially used at present, is about 500-1,500 hr, and the life depends on the amount of the gallium stored in the reservoir and the evaporation amount of the gallium on flashing.
The flashing operation is carried out by means of feeding electrical current into the heater or applying voltage across the heater for a certain period of time to obtain a desired temperature, referring to preliminarily determined current or voltage, which raises the temperature of the reservoir to 600-800xc2x0 C.
However, the gallium ion sources, which have been industrially used at present, have the life shortened for the reasons stated below when the gallium ion sources are subjected to the flashing operation at a constant certain current or voltage from the initial stage in use. Referring now to FIG. 3, there are shown changes in the temperature of the reservoir with time with respect to commercially available gallium ion sources having different remaining amounts of gallium, at the same current value. When the current value on flashing is constant, and when the flashing operation is carried out for a short period of time (for example, the period of time indicated by A in FIG. 3), the evaporation amount of the gallium increases to extraordinarily shorten the life of the gallium ion sources since the maximum temperature on flashing is raised with consumption of the gallium. Even when the period of time for flashing is set at the value indicated by B in FIG. 3, there is created a problem in that the evaporation amount of the gallium in each flashing operation becomes extremely great to shorten the life of the gallium ion sources due to a rapid consumption of the stored gallium since the reservoir temperature is kept high during most of the period for flashing. It has been proposed that the fed current, the applied voltage or the period of time on flashing be varied, depending on the remaining amount of the gallium, so as to avoid these phenomena for preventing the consumption of the gallium. However, the proposal has not provided sufficient improvement since it is practically difficult to estimate the remaining amount of the gallium properly. There is a great demand for a gallium ion source having stable flashing characteristics and stable life in terms of both ease in operation and high-availability of above-mentioned equipment utilizing gallium ion source.
It is an object of the present invention to provide a gallium ion source operated at a low temperature, capable of carrying out a flashing operation without changing a current or a voltage supplied to a heater, or a period of time for flashing operation, depending on the remaining amount of the gallium, for a flashing operation required to obtain stable ion emission, and consequently, to realize highly improved ease in operation and availability of equipment utilizing gallium ion source.
The inventors have conducted various experimental researches to find a solution for the object. The inventors have attained the present invention, finding that the phenomena stated earlier occur because the rate of the heat capacity of the gallium to the total heat capacity of the reservoir, the needle electrode and the gallium as heated portions in the ion source is great, and because the consumption of the gallium brings about a decrease in the heat capacity of the heated portions in the ion source, and hence to decrease the amount of heat required to bring the temperature of the heated portions to a steady state on flashing.
The present invention provides a gallium ion source comprising a needle electrode, a reservoir for storing gallium, and a wire heater for heating the needle electrode and the reservoir, wherein the needle electrode, the reservoir and the gallium stored in the reservoir have total heat capacity of not less than 0.015 J/K and not higher than 0.1 J/K. It is preferable that the needle electrode and the reservoir have total heat capacity of 0.01-0.099 J/K, that the gallium has an initial amount of 0.004-0.10 g, that the wire heater has a diameter of 0.10-0.20 mm and a length of 8-30 mm, and that the wire heater is formed in a V character shape so as to be bent at an angle of 40-90xc2x0. In the gallium ion source having such a specific range of heat capacity, the reservoir may comprise a coiled metallic wire provided in at least a dual structure. In the present invention, the heat capacity is in the specific range, and the needle electrode, the wire heater and the reservoir may be made of any material. However, it is preferable that the needle electrode, the wire heater and the reservoir are made of metal, such as tungsten and a tungsten-rhenium alloy, in terms of wettability and reactivity with gallium.
The present invention also provides a gallium ion source comprising a needle electrode, a reservoir for storing gallium, and a heater for heating the needle electrode and the reservoir and for sandwiching the reservoir therebetween, wherein the needle electrode, the reservoir and the gallium stored in the reservoir have total heat capacity of not less than 0.035 J/K and not higher than 0.1 J/K. It is preferable that the needle electrode and the reservoir have total heat capacity of 0.02-0.099 J/K, or that the gallium has an initial amount of 0.004-0.10 g.
The gallium ion source according to the present invention is useful to highly improve ease in operation and availability of equipment utilizing gallium ion source since the gallium ion source according to the present invention can minimize variations in the life without changing the heater current, the heater voltage or the period of time on flashing, depending on the remaining amount of the gallium and since the gallium ion source can provide a gallium ion beam having a long life.